Observations on hearts of patients in whom one or more aorto-coronary bypass conduits have been inserted; has been a continuing study in this unit. Twisting of an aorto-coronary bypass conduit was observed at necropsy in each of three patients. Three twists (540 degress just distal to the aortic anastomosis caused total obstruction of the bypass conduit to the left anterior descending coronary artery in 1 patient. A single twist (180 degrees occurred in a bypass conduit in each of the other 2 patients. Obviously, prevention of graft twisting is essential for successful bypass grafting.